The present invention relates generally to a method and apparatus for measuring the performance of an optical element, and more particularly to a point diffraction interferometer (“PDI”) that uses a wave front generated from a pinhole as a reference spherical wave in measuring a wave front of a projection optical system that transfers a mask pattern onto an object, and an exposure apparatus and method using the PDI. The inventive PDI is suitable, for example, for an apparatus for measuring the optical performance of the projection optical system that uses the soft X-ray or the extreme ultraviolet light (“EUV”) as exposure light.
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern of a mask (reticle) onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in the photolithography technology. The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to the wavelength of the exposure light, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Along with recent demands for finer processing to the semiconductor devices, a shorter wavelength of ultraviolet light has been promoted from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to a KrF excimer laser (with a wavelength of approximately 248 nm), an ArF excimer laser (with a wavelength of approximately 193 nm), and finally the EUV light having a wavelength between about 10 nm and about 15 nm.
Due to the extremely large light absorption in a material in a wave range of the EUV light, an exposure apparatus that uses the EUV light (or an “EUV exposure apparatus”) typically uses a catoptric optical system. The reflective optical element typically uses a multilayer coating mirror that alternately layers two types of materials having different optical constants, and a multilayer coating mirror in the projection optical system needs very high precision for its surface shape. Equation 1 below derived from the Marechal's criterion gives a permissible shape error σ (rms value), where n is the number of multilayer coating mirrors in the projection optical system, and λ is a wavelength of the EUV light:
                    σ        =                  λ                      28            ×                          n                                                          [                  EQUATION          ⁢                                          ⁢          1                ]            
For example, six multilayer coating mirrors in the projection optical system that uses the EVU light with a wavelength of 13 nm is permitted to have a shape error σof 0.2 nm. The permissible wave front aberration amount for the entire projection optical system is about 0.4 nm for the resolution of 30 nm pattern transferring. The conventional surface precision measuring apparatus is insufficient in precision to measure such a highly precise surface shape. Therefore, an application of the PDI that has a high measuring precision has been studied.
The PDI has been conventionally known as an apparatus that accurately measures a wave front aberration of the projection optical system. See, for example, Japanese Patent Application, Publication No. 57-64139, U.S. Pat. No. 5,835,217, and Daniel Malacara, “Optical Shop Testing”, John Wiley & Sons, Inc. 231 (1978). The PDI uses the light exited from a pinhole as a reference spherical wave, and the pinhole has a perfectly circular shape, because of the recognition and experience that the perfectly circular pinhole is likely to generate an ideal spherical wave. Since it is sufficient that the pinhole is shaped like a perfect circle when viewed from the incident direction, the pinhole can be shaped like an ellipse when it is inclined to the incident direction. See, for example, Japanese Patent Application, Publication No. 2-228505. Use of a circular polarization for the light incident upon the pinhole is also proposed rather than the linear polarization in order to reduce or eliminate the wave front error caused by the polarization state. See, for example, Japanese Patent Application, Publication No. 2001-227909.
As pointed out in Japanese Patent Application, Publication No. 2001-227909, the wave front exited from the pinhole (“pinhole exited wave front”) fluctuates according to polarization directions of the light incident upon the pinhole, offsets from the ideal spherical wave, and cannot maintain the predetermined measurement precision. As pointed out in Japanese Patent Application, Publication No. 2001-227909, the incident light upon the pinhole approaching to the circular polarization would reduce the polarization caused error of the pinhole exited wave front. However, the optical element other than the pinhole cannot maintain a constant polarization state due to the polarization dependency, such as a reflection and refraction, or provide precise measurements. In other word, a method disclosed in Japanese Patent Application, Publication No. 2001-227909 cannot necessarily obtain the highly precise measurements of about 0.1 RMS.